Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 081003
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
303 AM MST Thu Dec 8 2016

.DISCUSSION...

Today through Saturday...Northwesterly mid- to upper-level flow and
the arctic surface high pressure ridge will persist over our CWA
this morning. During this afternoon and evening, still expect
diffluent southwesterly flow aloft and associated lift ahead of a
low- to midlevel shortwave trough approaching from the Pac NW to
allow surface pressure falls to commence over the forecast area.
Also, a surface warm front accompanying this disturbance should move
northward into Southwest MT by late morning and then near the I-90
corridor by daybreak Friday. This pattern will permit weak
isentropic lift of warm, moist air to occur over the arctic air,
ahead of the front, and set the stage for a widespread snowfall this
evening into Friday morning. This will especially be the case over
our Southwest and Central MT counties, where the isentropic lift
looks to have a higher moisture content.

After daybreak on Friday, snow looks to become more scattered over
the CWA for a time as the aforementioned disturbance weakens while
moving farther eastward and flow aloft becomes more zonal due to a
rather flat mid to upper-level high pressure ridge building-in from
the Pac NW. This shift in the flow aloft will cause isentropic lift
to end over our area. Thus, upslope flow looks to become the main
trigger for snow. On Friday, the aforementioned warm front should
move farther northward toward the Central MT mountains. With
southerly surface flow south of the front and mainly northeasterly
to easterly flow north of the front, places such as the south
aspects of Southwest MT mountains and the north/east aspects of the
Rocky Mountain Front and Little Belts will likely experience
scattered upslope snow showers. By Friday afternoon, more widespread
snow should begin to overspread the region from the west as another
low-to midlevel shortwave trough approaches from the Pac NW.

As this disturbance continues eastward, the widespread snowfall will
persist Friday night into Saturday morning. By Saturday afternoon,
the best potential for lingering snow showers will be along the
Continental Divide and over the Southwest MT mountains due to
upslope flow. Elsewhere, quieter conditions are expected due to a
shortwave high pressure ridge building-in from the Pac NW. At the
surface, the aforementioned warm front is expected to stall near the
Central MT mountains Friday night into Saturday. Thus, this front
will continue to separate arctic air to its north from milder air to
its south. At this time, do not see any reason to adjust the winter
weather advisory that takes effect later today and continues into
Saturday for essentially the southern two thirds of the CWA. Snow
totals during the period should range from 1-4 inches on the plains
and in the valleys to mainly 4-6 inches in the mountains of
Southwest MT and along the Rocky Mountain Front. However, localized
totals of 6-10 inches cannot be ruled-out along the Rocky Mountain
Front and Southwest MT mountains.

As for temperatures, a slow moderating trend is expected in North-
Central MT between today and Saturday. However, temperatures will
still be about 15 to 30 degrees below normal during the period. In
Southwest MT, temperatures will be about 20 to 30 degrees below
normal today. By Saturday, temperatures here are expected to be near
normal.
Jaszka

Saturday Night through Wednesday...Flow aloft across Central and
Southwest Montana will generally be from the west Saturday night but
a shortwave moving through the flow will bring increased chances of
precipitation to the region by Sunday. The passage of the shortwave
will bring an associated cold front across the Canadian border by
late Sunday, and temperatures will take a downward direction again.
Another disturbance will move into the area Monday and this will
keep cold air across northern portions of the region. Snow will be
moving out of the area by Tuesday but the cold air already in place
will result in continued cold temperatures. High pressure will build
over the zones and the air mass will dry until another shortwave
brings snow across the Rockies again late Wednesday night and
Thursday. The surface boundary between cold air north and warmer
south will likely be lying across Southwest Montana and this area
would be expected to receive the most snow.
Zelzer

&&

.AVIATION...Updated 0530Z.
VFR conditions expected overnight and through Thurs as weak high
pressure ridging moves into central MT. Current area of mid-level
(5000-6000 ft) overcast over KGTF/KHLN/KBZN has shown little change
in areal coverage past several hours.  However, infrared satellite
imagery is showing a few breaks in the cloud deck and a patch of
drier west of the Continental Divide that may make its way into
central MT overnight further decreasing the cloud cover.  So have
kept mention of just scattered clouds for KBZN/KHLN/KGTF for
tomorrow morning, but confidence is only marginal at best on the
that cloud forecast.  Winds will remain light and variable across
the region through Thurs morning.

Have added mention of -SN and mix of VFR/MVFR ceilings for
KBZN/KEKS/KHLN/KGTF on Thurs night as broad plume of moisture from
next Pacific weather system moves into southwest MT and steadily
spreads northward.
Waranauskas

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF   0  -7   3  -1 /  10  40  40  60
CTB  -4 -11  -4  -7 /  10  20  40  50
HLN   7  -2  19  11 /   0  50  30  60
BZN  12   4  26  19 /   0  40  30  50
WEY  11   6  27  16 /  20  60  50  70
DLN  13   9  33  21 /  10  30  20  40
HVR   0  -7   1  -3 /  10  10  40  50
LWT   4  -6   6   0 /  10  30  40  60

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 11 PM this evening to 5 PM MST
Saturday Broadwater...Cascade...Central and Southern Lewis and
Clark...Fergus...Jefferson...Judith Basin...Meagher.

Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
Saturday Beaverhead...Gallatin...Madison.

&&

$$

weather.gov/greatfalls


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